Docs should mention homebrew as an install option

[anderseknert]

@jimmyraywv pointed out in the OPA Slack that the docs used to cover homebrew as an option for installing OPA. We currently only explain how to download the binaries, and while I'm sure many developers wil try brew install opa on their own, I think the docs should include this to show it's a supported option that as opposed to downloading a binary also helps keep OPA up to date.